Thick plate steel structure is widely used in the construction machinery, pressure vessels, ships, and other manufacturing fields. To obtain an acceptable welding quality and efficiency, thick plate steel is always joined by laser-arc hybrid welding technology. In this paper, Q355B steel with a thickness of 20 mm was taken as the research object, and the process of narrow-groove laser-arc hybrid welding was studied. The results showed that the laser-arc hybrid welding method could realize one-backing and two-filling welding with the single-groove angles of 8–12°. At different plate gaps of 0.5 mm, 1.0 mm, and 1.5 mm, the shapes of weld seams were satisfied with no undercut, blowhole, or other defects. The average tensile strength of welded joints was 486~493 MPa, and the fracture position was in the base metal area. Due to the high cooling rate, a large amount of lath martensite formed in heat-affected zone (HAZ) and this zone exhibited higher hardness values. The impact roughness of the welded joint was almost 66–74 J, with different groove angles.
